Chapter 1: Where are the electrons ?

Orbital Filling

Review VIDEO

The electron configurations of atoms are controlled by key principles that guide how the electrons "fill-up" orbitals. Note that the same principles also apply to electrons in orbitals in molecules:

Aufbau (Building-Up) Principle.  Electrons are placed into the atomic orbitals filling from bottom to top (like water in a jug) from lowest to highest energy (i.e. most stable to least stable).
The Schrodinger equation dictates the energy order of the orbitals: (but they can be readily determined by looking at the periodic table or using a simple mnemonic).

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p etc...... (it's not worth memorising this list, it's easier to use the mnemonic.)

(For the elements we use most of the time in organic chemistry, C, N and O, this should be easy as it is just the 1s 2s 2p part).

In simple terms, the Aufbau principle means fill the orbitals from bottom to top.

Hund's rule of Maximum Multiplicity.  If orbitals of equal energy (i.e. degenerate orbitals) are being filled, the electrons are placed into each orbital in the degenerate set before they are spin paired with another electron in the same orbital

In simple terms, Hund's rule requires single occupancy before pairing.

Incorrect violates Hund's rule		Correct satisfies Hund's rule

Pauli Exclusion Principle.  No two electrons in a atom can have an identical set of four quantum numbers. This means an orbital can hold a maximum of two electrons, and then the electrons must have opposite spins, +1/2 and -1/2.

In simple terms, the Pauli exculsion principle paired electrons to have opposite spins.

Incorrect violates the Pauli exclusion principle		Correct satisfies the Pauli exclusion principle

You should now try some more questions.

© Dr. Ian Hunt, Department of Chemistry